Imaging systems using spatial frequency transforms to process echo signals from real and synthetic aperture are now well-known in the art. Systems for processing data from such arrays have been described in publications such a C. N. Klahr, U.S. Pat. No. 3,805,224 issued May 20, 1979 and G. W. Adams et al., U.S. Pat. No. 4,717,916, issued Jan. 5, 1988.
While synthetic aperture arrays have been used, e.g., in certain radar systems, however, resolution has not been as high as desired due in part to errors introduced in computerized algorithms used to process the radiation data (from the echo signals) which is collected into a form for display.
Currently, stationary B-scanners are used, which are inserted into the body through the esophagus or colon, for chest cavity and pelvic cavity imaging, respectively. A B-scanner's resolution, however, is limited by its aperture size. A large B-scanner can be inserted into the body via the throat, but the procedure presents patient risk. Moreover, the depth of insertion into the colon for a large B-scanner is very limited. Imaging systems using small scanners which can readily be inserted into the body channels, such as via the esophagus, which allow high resolution with low patient risk are facilitated by this invention.
Acoustical imaging is a viable tool for examining ocean floors where optical images are not practical. Acoustic sources mounted on mobile robots or boats have been used for this purpose. Similar imaging geometrics also arise in geophysical exploration with a mobile transducer. This invention is useful to provide higher resolution imaging in such acoustical imaging applications even with small aperture arrays.
Synthetic aperture echo imaging has become viable for radar application, when the array's aperture is much smaller than the object's range. Complex computer processors have attempted to use background signals to synthesize the effect of a large aperture antenna. In a similar fashion, Inverse Synthetic Aperture Radar (ISAR) imaging utilizes the motion of the object to synthesize a large aperture system.
The far field radiation pattern of a radar resembles a spherical wave. The existing SAR/ISAR inverse methods are based on approximations for the spherical radiation pattern of the radar, e.g., the Fresnel approximation (stripmap-mode SAR), or the plane wave approximation (spotlight-mode SAR; ISAR imaging of a rotating object). These methods fail to provide accurate image data for high-resolution radar imaging (small wavelength and larger synthetic aperture) of a large object area. In synthetic aperture echo imaging problems of diagnostic medicine, sonar and geophysical exploration, the Fresnel approximation-based and the plane wave approximation-based methods can result in severely degraded and erroneous (scrambled) images of the object's (target) reflectivity function. The cross-range resolution can be improved when the temporal frequency of the radar's signal is increased. However, the phase error introduced in approximating a spherical wave by a plane wave is also an increasing function of the temporal frequency. Moreover, the inversion methods for reconstructing from an arbitrary set of straight line integrals are not applicable for reconstructing from line integrals over arbitrary curves, such as circular paths that arise in synthetic aperture echo imaging.